1. Field of the Invention
This invention relates to a new process for coating a substrate with high-temperature superconductive metal oxides, and more specifically to a process for coating non-reactive wire or other substrate of high melting point by dipping into a mixture of molten metal oxide.
2. Description cf the Prior Art
Superconductivity, the ability of a material to carry an electrical current without resistance, has been known since 1911. Superconductivity is observed below a critical temperature, the superconducting transition temperature, T.sub.c, which is specific to each material. Above their superconducting transition temperature, materials have a finite electrical resistance. For many decades, the highest known superconducting transition temperature was 23K. Ever since the discovery by Bednorz and Muller in 1986 of a compound having a superconducting transition temperature of 35K, much has been devoted to finding superconducting materials having even higher superconducting transition temperatures. C. W. Chu discovered a composite having a transition temperature of 90K, and more recently, a composition having a transition temperature of 125K has been found. Any transition temperature above 30K is referred to high temperature. The rationale behind this effort is that the higher the superconducting transition temperature, the lower the refrigeration energy required to maintain the material in a superconducting state, and the less expensive it is to operate superconducting devices.
There are three principal high temperature superconductor compositions; yttrium-barium-copper oxide, with the metals in the atomic ratio of 1:2:3, referred to as "1-2-3", with a critical temperature T.sub.c of 93K; thallium-barium-calcium-copper oxide, with T.sub.c =125K; and bismuth-based copper oxides with T.sub.c as high as 110K The 1-2-3 composition has a relatively low T.sub.c and the product tends to be brittle. Thallium-containing compositions are unpopular because of the toxicity of thallium. Bismuth-containing compositions are believed to be the best of the three groups of compositions in that the T.sub.c is above 100K and they are easiest to process.
Following are examples of prior-art processes and compositions.
Japan Appl. No. 62-166098, published 13 Jan. 1989, entitled MANUFACTURE OF SUPERCONDUCTIVE FILM. A nitrate-mixed solution containing at least y, Ln, Ba, Sr, and Cu is coated on a substrate to form a superconductive film.
Japan Appl. No. 62-166499, published 13 Jan. 1989, entitled MANUFACTURE OF SUPERCONDUCTIVE THIN FILM discloses immersing a substrate in a molten solution of compounds containing group IIa and group IIIa and Cu whereby the molten solution sticks to the substrate to form a thin film. The substrate is then lifted out and cooled in the atmosphere and annealed at 800.degree.-950.degree. C. to form a ceramic superconductive thin film coating.
PHYSICS TODAY, pp. 17-25, April, 1988, discusses a basic building block of Bi-Ca-Sr-Cu-O (page 22) in the atomic ratio of 4:3:3:4. The product has two critical temperatures T.sub.c, namely 85K and 110K.
APPL. PHYS. LETT. 51 (12), Sept. 21, 1987, pp. 943-945, discloses a preform-wire melting process. A ceramic/metal core composite superconducting wire is formed by remelting the outer shell of a Ba-Y-Cu oxide and heat treating with oxygen; it is said to have improved critical current density.
U.S. Pat. No. 4,826,808 discloses methods of preparing superconducting oxide from metallic elements to form a composite oxide which is formed into products by various methods such as plasma sputtering, gas atomizing, chemical vapor deposition, as well as dipping; thermal mechanical processing includes drawing, extrusion, pressing and rolling. The dipping process is carried out in vacuum or an inert atmosphere.
The processes including drawing, sputtering, vapor deposition, etc., all require complicated manufacturing procedures and precise control of the atmosphere under which the composition is processed. Therefore there is a need for a simple, inexpensive process for producing a high temperature superconducting material meeting specific industrial requirements.